The first solar cells were fabricated in the mid 1950s from crystalline silicon wafers. At that time, the most efficient devices converted 6% of solar power to electricity. Advancements in solar cell technology over the past 50 years have resulted in the most efficient Si cell being at 25% and commercial Si modules being at 10%.
Despite these efficiencies, the high cost of manufacturing conventional solar cells limits their widespread use as a source of power generation. The construction of conventional silicon solar cells involves four main processes: the growth of the semiconductor material, separation into wafers, formation of a device and its junctions, and encapsulation. For cell fabrication alone, thirteen steps are required to make the solar cell and of these thirteen steps, five require high temperatures (300° C.-1000° C.), high vacuum or both. In addition, the growth of the semiconductor from a melt is at temperatures above 1400° C. under an inert argon atmosphere. To obtain high efficiency devices (>10%), structures involving concentrator systems to focus sunlight onto the device, multiple semiconductors and quantum wells to absorb more light, or higher performance semiconductors such as GaAs and InP, are needed. These options all result in increased costs.
Another problem with conventional solar devices is the high cost of manufacturing materials. The amount of silicon needed for 1 kW of module output power is approximately 20 kg. At $20/kg, the material costs for electronic grade silicon are partially subsidized by the chip manufacturing sector. Other materials such as GaAs, which are synthesized with highly toxic gases, are a factor of 20 higher in cost at $400/kg. Because solar cells are large area devices, such material costs hinder the production of inexpensive cells. As a result, thin film devices, which have active layers several microns thick of amorphous Si, CdTe, and CuInSe2 are being explored. Also, in 1991, O'Regan et al. reported the invention of a novel photochemical solar cell comprised of inexpensive TiO2 nanocrystals and organic dye (O'Regan et al. Nature 353, 737 (1991)).
Embodiments of the invention improve upon such conventional devices and address the above problems individually and collectively.